They expect "explosive" storm growth over the next few hours with a moderate potential for strong tornadoes and severe winds. This watch is probably somewhat in response to the 18Z Davenport sounding:

That backing of the low-level wind field to more southerly flow has indeed happened, which is ramping up the low-level wind shear. We also see that the cap has rapidly eroded with steepening lapse rates in the low-levels. Sounding analogs (which look at previous soundings from severe weather events for similarities) show an 80% match with previous tornadic events. Definitely be on the lookout this afternoon

It looks like it could be a rough day for severe weather in northern Illinois. The Storm Prediction Center has issued a relatively rare "high risk" for northern Illinois and Indiana, including the Rockford and Chicago areas.

There has actually been a fair bit of uncertainty regarding the development and evolution of thunderstorms in this area over the past 24 hours. Yesterday several models were pointing to a powerful squall line (or quasi-linear convective system if you prefer that term) moving across northern Illinois early this morning. Take the NCAR EnKF-initialized 3km WRF forecast from yesterday morning--here's their forecast reflectivity over the upper midwest for 15Z this morning:

This suggested an intense derecho-type storm (note the bow-echo appearance in this simulated radar composite) that was to move across southern Wisconsin and northern Illinois this morning bringing strong winds and heavy rain. However, here's what the radar actually looked like at 15Z this morning:

No big thunderstorm complex---only some areas of rain. There were some thunderstorms earlier, but they were not nearly as strong as many of the models had suggested. We could tell even last night that this big derecho-type storm was not going to materialize as the precursor storms did not get their act together further west.

However, that brings us to today's forecast. There's a very big consequence of the lack of storms this morning--the entire purpose of thunderstorms is to re-stabalize the atmosphere by convectively removing pre-existing instability. Had we had thunderstorms move through this morning, they probably would have done just that--cooled and dried out the lower levels of the atmosphere a bit and leave us less unstable today. But that didn't happen. Now we get another day of heating an already conditionally unstable airmass and this will destabilize the air even more...setting up today's potential for severe weather.

Let's look at the setup this morning. A low pressure center has been getting its act together in the central Plains all night. Here's the surface mesoanalysis from 12Z this morning:

You can see the cluster of storms that was there earlier, though, as I said, not nearly as strong as some models were indicating. If we look at the Davenport sounding from this morning we see reasons for concern:

You can click on that image to make it larger. One of the striking features of this sounding is actually the wind profile--almost uniform wind direction all the way through the troposphere. This enhances the possibility of these derecho/straight-line wind events as storms will quickly tend to organize in a line perpendicular to a uniform wind profile. The surface dewpoint this morning is in the low 60s in Davenport and predicted to rise. We also see relatively high dewpoints all the way up to around 850mb and near saturation above 500mb (that may have something to do with outflow aloft from the storms to the north. All in all this sounding shows a lot of moisture in the air--meaning heavy rain is definitely a possibility.

We also have a fair bit of instability. The sounding profile between about 800mb and 500mb shows a pattern indicative of what is called an "elevated mixed layer" (EML). This layer of the atmosphere is marked by extremely steep lapse rates (temperatures falling off rapidly with height--basically dry adiabatic) and well-mixed moisture content. This actually leads to this layer being relatively dry--you can see that the dewpoint is well below the temperature for much of this layer. The presence of an EML enhances the potential for very strong downdrafts--all that moisture being lofted in the thunderstorm can evaporate into that dry layer, causing a lot of cooling within that layer. This cooled air becomes negatively buoyant and sinks down to the ground, creating powerful downdrafts and organizing a "cold pool" beneath the storms. This kind of setup again favors organized, bow-echo sorts of storms.

Another concern is the potential for tornadoes. The wind profile in that sounding doesn't show a lot of low-level directional shear, but this will change as the surface low moves closer. Here's the surface analysis from 17Z this morning:

As the low has moved closer, notice that the surface winds have become more southerly across eastern Iowa and northern Illinois. This is creating more directional wind shear in the low-level wind profile. The low pressure center is also forecast to deepen today as we see a shortwave trough approaching from the west in the 500mb analysis, helping to give the low an added boost.

The convection-allowing models we have are a bit messy with the convection they are developing today. Here's the Composite Reflectivity forecast from the HRRR model for 23 Z this evening:

Lots of storms popping up all over the place. It's difficult to tell if there's any forecast organization. I ran WRF simulation last night with a few different parameters and had this forecast for 23Z tonight:

Similar picture with a lot of discrete storms across northern Illinois. It looks like the models are making any capping inversion relatively easy to break which may explain why we seem to have storms popping up everywhere. My WRF run has the storms starting to organize into bowing line segments east of Chicago around 3Z tonight:

All in all it looks like we're in for a crazy day weather-wise across northern Illinois and Indiana. We'll have to be watching to see how this develops.

Current UTC time

My personal weather station: KWASEATT252

UPDATE

About Me

Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
Email me at lukemweather@gmail.com.
http://www.midlatitude.com/lukemadaus